1) Field of the Invention
The present invention relates to an optical semiconductor module having an optical semiconductor element and an optical fiber that are used for optical communications.
2) Description of the Related Art
FIG. 23A is a plan view of a part of a conventional semiconductor laser diode (hereinafter, “LD”) module, and FIG. 23B is a longitudinal cross-sectional view of the part. The LD module includes a base 6 onto which a heat sink 3 and a carrier 5 are soldered. On the heat sink 3, an LD element 1 and a thermistor 2 are mounted, and on the carrier 5, a photo diode (hereinafter, “PD”) element 4 is mounted. Further, a metal ferrule 7a supporting an optical fiber 7 is fixed onto the base 6 via two support members 6a by YAG laser welding or with solder. The welding points, in the drawings, are denoted by solid circles Pwd. The optical fiber 7 is precisely aligned so as to be optically coupled with a laser beam emitted from the LD element 1, and is fixed onto the support member 6a. 
FIG. 24 is a longitudinal cross-sectional view of the conventional LD module, where the base 6 is accommodated within a package 8, and is fixed onto a bottom plate 8e of the package 8. The package 8 is sealed with a lid 9 at the upper opening, thereby to complete the assembly of the LD module. A lensed fiber, one end of which being formed in a spherical or a wedged shape to serve as a lens portion, is used as the optical fiber 7 in order to ensure a high coupling efficiency between the optical fiber 7 and the laser beam emitted from the LD element 1. The other end of the optical fiber 7 is led out of the package 8 through a longitudinal hole 8a of a snout 8b prior to mounting of the base 6 onto the bottom plate 8e of the package 8. A portion between the optical fiber 7 and the internal wall of the snout 8b is hermetically sealed with sealant S like solder or synthetic resin.
The LD module, particularly one using the lensed fiber, requires that the LD element 1 and the lensed fiber are coupled with an extremely high precision, and that the optical fiber 7 and the support member 6a are precisely positioned.
In the LD module shown in FIG. 24, the base 6 is directly fixed onto the bottom plate 8e, with no Peltier module for cooling interposed. The longitudinal hole 8a in FIG. 24 is formed through a front wall 8c of the package 8 at a position far down from the upper end of the front wall 8c. Such position of the longitudinal hole 8a makes the mounting of the base 6 difficult.
FIG. 25 is a longitudinal cross-sectional view of the conventional LD module in the mounting step. In the mounting step, the base 6 is accommodated onto the bottom plate 8e, maintaining a state that the optical fiber 7 passes through the longitudinal hole 8a. In such a situation, a position of the metal ferrule 7a holding the optical fiber 7 is limited by the position of the longitudinal hole 8a, causing often an interference between a lower portion of the base 6 and a rear wall 8d of the package 8 when mounting the base 6, depending on sizes of the package 8 and the base 6. And while the base 6 is being mounted onto the bottom plate 8e of the package 8 so as to avoid the interference, stress is often applied to the optical fiber 7 and thereby the optical fiber 7 is bent, pressed, or pulled. In this way, this stress often causes misalignment between the LD element 1 and the optical fiber 7.
Further, in the mounting step, the optical fiber 7 is often excessively bent at a portion A encircled by a dashed line in FIG. 25, by contact with the snout 8b. In some cases, the optical fiber 7 is coated with metal (e.g., gold), in order to facilitate the hermetic sealing of the optical fiber 7 inside the inner space of the snout 8b with solder S or other sealant or in order to facilitate the soldering of the optical fiber 7 to the base 6. Such metal-coated optical fibers have a minimum allowable bending curvature radius larger than that of non-coated optical fibers and are more repulsive against the bending deformation. Therefore, the optical fiber 7 needs to be handled so as not to be bent too much. The similar situation may be present not only in optical semiconductor modules in general of butterfly-type, including receiver modules (hereinafter, “PD modules”) having a PD element mounted on the base 6, but also in the LD module of so-called DIL (Dual In Line) type which utilizes a small-sized package.